Let's write a proper "Read Me First"

Question

Our site has a lot of rules.

One might even say, an awful lot.

It would be a very good idea to list the most relevant of them in one place somewhere (the tour seems like a decent option) so that we wouldn't need to give many a well-meaning newcomer a figurative kick in the face when they first try to post, and we explain exactly which meta posts and VTC reasons they should have presciently known in order to avoid the torrent of downvotes that resulted.

As answers, I'm hoping to see some concisely worded, basic rules on what one should post, and how. Along the lines of:

• Mostly, our questions are puzzles of our own making.
• Do not ever post someone else's puzzle as your own
• A puzzle should have a unique solution clearly identifiable as such.
• The tag wikis will sometimes contain a section on "what (not) to do"

and so on.

Having these clearly stated would make the first step into PSE a lot easier to take, I believe.

Answer 1

Since the question is getting a lot of support, but there are no answers yet, I'm starting this CW on Rand's idea: FAQify the posts in here and summarise the rules.

Since I lack the power to add the faq tags, I'm just writing some summaries here, grouped by the newcomers action for which they are relevant. Most links are still missing, as are probably some other conventions that it would be useful for a newcomer to know.

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This community wiki answer is a work in progress, and not yet complete.

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Asking:

• Our questions are mostly puzzles of our own invention.
  • Puzzles should have a unique answer identifiable as such.
  • Questions that look like maths homework will be closed.
• Asking questions related to puzzles and puzzle design is also ok.
• Respect copyrights. Only use material you have a permission to use.
  • If you post a puzzle you didn't create yourself, always attribute the original author.
• The common practise is to accept the first correct answer, but you are free to accept any answer by whichever criteria you like.
• Avoid major changes to questions after they have been answered.
  • Interactive puzzles are very limited, if not discouraged as a result.
• It's ok to add hints to questions, but waiting 24 hours before doing so is appreciated.
• Some tag wikis provide good advice on what (not) to do when asking. Typical examples include "What is a XYZ™ item", "Number Sequences" and "Code Puzzles". TODO: Add more examples

Answering:

• An answer to a puzzle should always explain why it's the correct answer.
• Partial answers are ok, but not always so.
• Use spoiler tags >! at the start of lines to hide the crucial parts of the answer from accidental views.

Commenting:

• If you have a minor improvement to an existing answer, add a comment rather than a new answer.
• Rot13 encryption is sometimes used to hide possible spoilers in comments. @Alconja's nice script can encrypt/ decrypt rot13 upon mouseover.

Voting:

• Use upvotes and downvotes as you please. When downvoting, adding a constructive comment is much appreciated.

Other:

• A collection of helpful tools to assist you can be found here.
• If you need help, the easiest way is to ask at The Sphinx's Lair.